1. Field of the Invention
The present invention relates to an electric power steering system that can conduct an appropriate fail-safe processing by setting an upper limit of a current or voltage which is supplied to a motor and controls the motor which is improved in steering feeling.
2. Description of the Related Art
FIG. 15 is a block diagram schematically showing the structure of a controller in a conventional electric power steering system as disclosed in Japanese Patent Unexamined Publication No. Hei 8-175404.
The conventional electric power steering system includes a power source in the form of a motor 1 for assisting a steering force or effort of a driver, a torque sensor 2 for detecting the steering force upon steering of the driver, and an input interface (hereinafter simply referred to as "I/F") 3 for inputting a detection signal from the torque sensor 2 to an A/D converter 21 which will be described later. The interface 3 is designed to conduct d.c. amplification and phase compensation.
The electric power steering system 1 also includes a vehicle speed sensor 4 for detecting a travel speed of a vehicle, an input I/F 5 for inputting a signal from the vehicle speed sensor 4 to an I/O port 20 which will be described later, a motor current detector circuit 6 for converting a current (i.e., motor current) supplied to the motor 1 into a predetermined voltage so as to input it to an A/D converter 21 which will be described later, and a motor terminal voltage detector circuit 9 for converting a voltage level of a terminal voltage of the motor 1 and inputting the converted terminal voltage to the A/D converter 21, the motor terminal voltage detector circuit 9 also having a low pass characteristic that allows a frequency band lower than a PWM carrier frequency for driving the motor 1 to pass therethrough.
The motor current detector circuit 6 includes a current detection resistor 7 that converts the motor current into a voltage, and an amplifier circuit 8 for amplifying a voltage across the current detection resistor 7.
The motor terminal voltage detector circuit 9 includes resistors 10a to 10f and capacitors 11a, 11b.
Also, an H-type bridge circuit 12 for driving the motor 1 is made up of power MOSFETs 13a to 13d.
A driver circuit 14 is made up of FET drivers 14a to 14d for driving the power MOSFETs 13a to 13d, respectively. Also, a microcomputer 15 that serves as a controller includes a CPU 16, a ROM 17 for storing therein a control program, etc., a RAM 18 for temporarily storing data and the like therein, a PWM timer 19 for generating a pulse signal to drive the motor 1, an I/O port 20a, an I/O port 20b, an A/D converter 21 and a timer 22 which is used for management of a control period, etc.
The PWM timer 19 is connected with input terminals of the FET driver 14a and the FET driver 14c.
Also, the I/O port 20a is connected with the FET drivers 14b and 14d, and similarly, the I/O port 20b is connected with the vehicle speed sensor 4 through the vehicle speed sensor input I/F 5.
The A/D converter 21 is connected with output terminals of the motor current detector circuit 6 and the motor terminal voltage detector circuit 9, and also connected with an output terminal of the torque sensor 2 through the torque sensor input I/F 3.
FIG. 16 is a functional block diagram showing the controller in the conventional electric power steering system shown in FIG. 15.
As shown in FIG. 16, the controller 15 functionally includes a steering force assisting current arithmetic operating means 23, a current control means 24, a motor angular speed arithmetic operating means 25, a motor angular acceleration arithmetic operating means 26, a Coulomb's friction compensating current arithmetic operating means 27, a viscosity friction compensating current arithmetic operating means 28, and an inertia compensating current arithmetic operating means 29. The functions of those means are obtained when the CPU 16 executes the control program stored in the ROM 17.
The assisting current arithmetic operating means 23 that arithmetically operates or calculates the motor current for statically assisting the steering force is designed to arithmetically operate or calculate a steering force assisting current target value Is on the basis of a detected steering force value Vt corresponding to a steering force which has been detected by the torque sensor 2 and a detected vehicle speed value Vs corresponding to a vehicle speed which has been detected by the vehicle speed sensor 4.
The current control means 24 for controlling a current flowing through the motor 1 is designed to conduct feedback control in such a manner that a detected motor current value Ia.sup.sns, which has been detected by the motor current detector circuit 6, coincides with a motor target current Ia1, and to arithmetically operate or calculate a voltage Va1 which is applied to the motor 1.
The motor angular speed arithmetic operating means 25 for arithmetically operating or calculating the angular speed of the motor 1 is designed to arithmetically operate or calculate a motor angular speed estimate .omega. on the basis of a motor terminal voltage Va.sup.sns, which is obtained from motor terminal voltages V12 and V22 detected by the motor terminal voltage detector circuit 9, and the motor current Ia.sup.sns detected by the motor current detector circuit 6.
The motor angular speed arithmetic operating means 26 is designed to arithmetically operate or calculate a motor angular acceleration estimate d.omega./dt on the basis of the motor angular speed estimate .omega. which has been arithmetically operated or calculated by the motor angular speed arithmetic operating means 25.
The Coulomb's friction compensating current arithmetic operating means 27 arithmetically operating or calculating the motor current for compensating the Coulomb's friction of a steering system is designed to arithmetically operate or calculate a Coulomb's friction compensating current target value Ic on the basis of the motor angular speed estimate .omega. and the vehicle speed Vs which has been detected by the vehicle speed sensor 4.
The viscosity friction compensating current arithmetic operating means 28 arithmetically operating or calculating the motor current for compensating the viscosity friction of the steering system is designed to arithmetically operate or calculate a viscosity friction compensating current target value Id on the basis of the motor angular speed estimate .omega. and the vehicle speed Vs which has been detected by the vehicle speed sensor 4.
The inertia compensating current arithmetic operating means 29 arithmetically operating or calculating the motor current for compensating the inertia moment of the steering system is designed to arithmetically operate or calculate an inertia compensating current target value Ij on the basis of the motor angular acceleration estimate d.omega./dt and the vehicle speed Vs which has been detected by the vehicle speed sensor 4.
Now, the operation of the above-mentioned conventional electric power steering system will be described.
When a steering wheel is steered to generate a steering torque in the steering system, the torque sensor 2 detects the steering torque and output a corresponding voltage value Vt to the CPU 16 through the A/D converter 21.
After this, processing is conducted according to the control program, and the steering force assisting current arithmetic operating means 23 arithmetically operates or calculates the steering force assisting current target value Is, for example, as shown in FIG. 17, on the basis of the detected vehicle speed value Vs and the detected steering force value Vt to supply its result to the current control means 24 as the motor target current Ia1.
The current control means 24 conducts feedback control so as to make the detected motor current value Ia.sup.sns coincide with the motor target current Ia1, arithmetically operates the voltage Val which is applied to the motor 1, and in order to apply Va1 to the motor 1, gives signals from the PWM timer 19 and the I/O port 20a to the H-type bridge circuit 12 to drive the motor 1.
Upon driving the motor 1, the voltages V12 and V22 at the respective terminals of the motor 1 are detected by the motor terminal voltage detector circuit 9, and then inputted to the CPU 16 through the A/D converter 21.
After that, processing is conducted according to programs, and the CPU 16 arithmetically operates or calculates the detected motor terminal voltage value Va.sup.sns as Va.sup.sns =V12-V22, and then gives this value Va.sup.sns to the motor angular speed arithmetic operating means 25.
The motor angular speed arithmetic operating means 25 arithmetically operates or calculates the motor angular speed estimate .omega. on the basis of the detected motor terminal voltage value Va.sup.sns and the detected motor current value Ia.sup.sns, and gives this value .omega. to the motor angular acceleration arithmetic operating means 26, the Coulomb's friction compensating current arithmetic operating means 27 and the viscosity friction compensating current arithmetic operating means 28.
The motor angular acceleration arithmetic operating means 26 obtains the motor angular acceleration estimate d.omega./dt by differentiating the motor angular speed estimate .omega., and gives this value d.omega./dt to the inertia compensating current arithmetic operating means 29.
The Coulomb's friction compensating current arithmetic operating means 27 arithmetically operates or calculated the target value Ic of the Coulomb's friction compensating current from the vehicle speed Vs and the motor angular speed estimate .omega., for example, on the basis of a characteristic shown in FIG. 18, and then adds the target value Ic to the steering force assisting current target value Is.
The Coulomb's friction compensating current target value Ic is to improve the return of the steering wheel which is particularly deteriorated at low speed.
The viscosity friction compensating current arithmetic operating means 28 arithmetically operates or calculates the target value Ia1 of the viscosity friction compensating current from the motor angular speed estimate v.omega., for example, on the basis of a characteristic shown in FIG. 19, to add it to the above steering force assisting current target value Is.
The target value Id of the viscosity friction compensating current gives a viscosity feeling to the driver's steering feeling, and also improves the convergence when returning the steering wheel to a home or neutral position, which would be degraded particularly at high speed.
The inertia compensating current arithmetic operating means 29 arithmetically operates or calculates the target value Ij of the inertial compensating current that has an effect of reducing such an inertia feeling that the steering force is increased particularly at the time of reversing a steering direction due to an influence of the inertia moment of the motor 1, from the motor angular speed estimate d.omega./dt, for example, on the basis of a characteristic shown in FIG. 20, and then adds it to the above steering force assisting current target value Is.
In this way, the motor target current Ia1 is obtained from the following equation based on the steering force assisting current target value Is, the Coulomb's friction compensating current target value Ic, the viscosity friction compensating current target value Id and the inertia compensating current target value Ij. EQU Ia1=Is+Ic+Id+Ij (1)
The value Ia1 thus calculated is given to the current control means 24 so as to control the motor current, likewise.
Now, the motor angular speed arithmetic operating means 25 will be described in more detail.
Provided that the motor 1 is a separately excited d.c. motor, an equivalent circuit of an armature can be expressed as shown in FIG. 21. In this figure, Ra is an armature resistor, La is an armature inductance, Ve is a motor inductive voltage, Va is a motor terminal voltage, and Ia is a motor current.
In FIG. 21, if transient terms based on the armature inductance La are ignored, the following equation is satisfied. EQU Va=Ia.times.Ra+Ve (2)
Here, Ve is expressed as follows. EQU Ve=Ke.times.m (3)
where Ke is a motor inductive voltage constant, and m is a motor angular speed.
From the equations (2) and (3) above, the following equation is obtained. EQU m=(Va-Ia.times.Ra)/Ke (4)
Since Ra and Ke are constants, and since Va and Ia can be replaced by detected values, a motor angular speed estimate .omega. can be calculated by the following equation. EQU .omega.=(Va.sup.sns -Ia.sup.sns .times.Ra)/Ke (5)
where Va.sup.sns is a detected motor terminal voltage value, and Ia.sup.sns is a detected motor current value).
Thus, with the conventional electric power steering system as structured above, the motor angular speed estimate is arithmetically operated or calculated on the basis of the motor terminal voltage.
For this reason, if the motor terminal voltage detector circuit is in failure, then the motor angular speed estimate is not correctly arithmetically operated whereby a current is allowed to flow in the motor regardless of the steering state of the driver. As a result, there arises such a problem that abnormality takes place in the control of the steering force by means of the electric power steering system.